Composite foils such as electrodeposited copper foils with carrier foil have been employed as a material for producing printed circuit boards, which are widely used in the electric and electronic industries. In general, the composite foil is bonded, through hot pressing, onto an electrically insulating polymeric substrate such as glass epoxy substrate, phenolic polymer substrate, or polyimide, and the carrier foil is subsequently removed to form a copper-clad laminate.
Using composite foils for preparing copper clad laminates is very advantageous as it protects the surface of the electrodeposited foil against dust, tears and wrinkles during handling and hot-press-forming.
Composite foils are generally divided into two types: i.e. foils with peelable carriers and foils with etchable carriers. Briefly, the difference between the two types of composite foils lies in the method of removing the carrier foil after completion of hot-press-forming. In peelable composite foils, the carrier foil is removed by peeling, whereas in etchable composite foils, the carrier foil is removed by etching.
Peelable composite foils are generally preferred to etchable composite foils as they allow simpler and more precise preparation of copper clad laminates. Indeed, chemical etching of the carrier is long, due to its relatively important thickness, requires several changes of etching baths and results in a rough surface. In addition, it limits the choice in carrier foil since the ultra-thin foil must not be etched.
Peelable composite foils are thus much easier to use than etchable foils. However, a recurrent problem of conventional peelable composite foils is the difficulty of controlling their peel strength, i.e. the force needed to separate the carrier foil from the electrodeposited copper foil. Indeed, during hot-press-forming the peelable composite foil is submitted to high temperatures, which tends to increase the adhesion of the carrier foil and leads to considerable variations in peel strength. In some cases, a carrier foil cannot be removed from the copper clad laminate.
A particularly interesting development in composite foils was made to comply with the actual needs of the electronic industry. Indeed, as electronic equipment becomes smaller and lighter with higher performance, it is necessary to reduce the width of wiring lines and the diameter of via holes which connect layers in multi-layer printed circuit boards (MLB). In order to make via holes of below 200 μm in diameter, generally called microvias, the use of lasers has been proposed.
WO 00/57680 describes a composite foil of the peelable type which is particularly suited to be used in processes for manufacturing multi-layer printed circuit boards, wherein microvias are drilled by means of a CO2 laser. This composite foil comprises a carrier foil, a release layer on one side of the carrier foil, and an ultra-thin copper foil—less than 10 μm thick—having a front side facing the release layer and an opposite back side coated with a resin. In order to improve the absorption of CO2 laser light, the front side of the ultra-thin copper foil has received a surface preparation, in particular to reduce the reflection of the laser light. Hence, after removal (peeling) of the carrier foil, the ultra-thin copper foil has a surface with a low reflectivity, whereby the conditions of laser drilling and thus the drilling speed and the quality of the microvias are improved.
Such a surface preparation of the front side of the ultra-thin copper foil is achieved during the manufacturing of the composite foil. It consists in giving the ultra-thin copper foil a surface colour favouring the absorption of CO2 laser light by forming a thin layer of dark coloured electrically conductive material over the release layer on the carrier foil, before electrodepositing the ultra-thin foil.
A first way of achieving such a surface preparation is carbon deposition. A liquid carbon dispersion, generally containing carbon, one or more surfactants capable of dispersing the carbon, and a liquid dispersing medium such as water, is applied to the side of the release layer which will be facing the ultra-thin copper foil. A dark layer of electrically conductive material is thus formed on the release layer, and the ultrathin copper foil is then electrodeposited on this dark layer.
Alternatively, the dark coloured electrically conductive layer can be formed by a dark coloured electrically conductive polymer. A monomer which is electrically conductive in its polymerised form, such as e.g. pyrrole, is applied to the surface of the release layer by a wet process. The monomer is thereafter polymerised, and the ultra-thin copper foil is electrodeposited over the polymer layer.
Despite the improvement provided by such a composite foil with regard to microvia drilling, the peel strength of such a composite foil is difficult to optimise.